Extracellular matrix (ECM) mineralization is a physiologic process in bone and teeth and a pathologic one everywhere else in the body. Pathologic ECM mineralization often has deleterious consequences in conditions such as coronary artery disease and osteoarthdtis two diseases for which no curative treatment is available. Despite the biological importance of ECM mineralization the molecular mechanisms restricting it to bone physiologically remain unknown. In particular gene deletion experiments failed to identify osteoblast-specific genes necessary to initiate bone mineralization while they demonstrated the existence of inhibitors of pathologic ECM mineralization. Two of these inhibitors, Npps and Ank, act by producing inorganic pyrophosphate, an inhibitor of ECM mineralization, and exporting it outside the cells. However, the observation that Npps and Ank are expressed in osteoblasts (data not shown), further complicates our understanding of bone mineralization. The absence of osteoblast-specific proteins initiating bone mineralization together with the expression in osteoblasts of mineralization inhibitors like Npps and Ank that affect phosphate metabolism led us to test the following hypothesis: could the spatial restriction of ECM mineralization to bone be explained, at least in part, by coexpression in osteoblasts of genes that are not osteoblast-specific but that affect phosphate metabolism and ECM assembly? The specific aims are: -To induce ectopic ECM mineralization by ectopic expression of tissue non-specific alkaline phosphatase -To raise TNAP levels in blood to determine if this alone can induce ectopic ECM mineralization -To determine whether ECM mineralization in the growth plate is a molecular determinant of longitudinal growth of the skeleton -To use genetic means to identify substrates for TNAP in bone -To address the role of calcium ions in the initiation of ECM mineralization. We believe that this analysis will greatly enhance our understanding of cartilage and bone ECMs mineralization. This may open novel research avenues to understand the mechanisms leading to the closure of the growth plate at the end of puberty. Finally, it could provide new therapeutic avenues to explore diseases characterized by abnormal or ectopic ECM mineralization such as osteoarthdtis (OA).